> ## Documentation Index
> Fetch the complete documentation index at: https://docs.cipherinspector.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Missing Characters

> Detect which characters from the expected character set are absent in the ciphertext.

## How it works

The Missing Characters widget analyzes your ciphertext to identify which characters from
the expected character set are not present. The expected character set depends on the
encoding type of your ciphertext.

<Steps>
  <Step title="Determine Character Set">
    Based on the ciphertext's encoding type, the widget identifies the expected
    character set:

    * **Text encodings** (UTF-8, ASCII, UTF-16, UTF-32, Latin1): Letters A-Z
    * **Data encodings**: Characters specific to that encoding format
  </Step>

  <Step title="Scan Ciphertext">
    The widget scans the original ciphertext string, not the decoded bytes, to find
    which characters from the expected set are present.
  </Step>

  <Step title="Report Missing">
    Characters that are not found in the ciphertext are reported as missing.
    Optionally, present characters can also be displayed.
  </Step>
</Steps>

## Character Sets by Encoding

### Text Encodings

For text-based encodings (UTF-8, ASCII, UTF-16, UTF-32, Latin1), the widget checks for
the presence of letters A through Z. The check is case-insensitive, meaning both
uppercase and lowercase letters count toward the same character.

### Data Encodings

| Encoding    | Expected Characters          |
| ----------- | ---------------------------- |
| Binary      | 0, 1                         |
| Octal       | 0, 1, 2, 3, 4, 5, 6, 7       |
| Decimal     | 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 |
| Hexadecimal | 0-9, A-F (case-insensitive)  |
| Base64      | A-Z, a-z, 0-9, +, /          |

## Settings

### Show Present Characters

When enabled, the widget displays both missing characters (in red) and present
characters (in green). When disabled, only missing characters are shown.

## Practical Applications

The Missing Characters widget can be used to:

* **Identify substitution cipher constraints**: If certain letters never appear in a
  ciphertext, they may have been deliberately avoided or mapped to other symbols
* **Detect encoding anomalies**: A hexadecimal string missing certain digits may
  indicate a pattern or constraint in the original data
* **Verify data completeness**: Check if a Base64 or hex dump contains all expected
  characters
* **Analyze cipher alphabets**: Determine if a cipher uses a reduced alphabet. For example, if one character is missing it may indicate a Playfair cipher that was then rotated, as Playfair traditionally replaces I/J.

## Caveats

* For text encodings, only letters A-Z are checked. Numbers, punctuation, and special
  characters are not included in the analysis
* For data encodings, the analysis is performed on the encoded string itself, not the
  decoded byte values
* Whitespace and formatting characters in the original text are ignored when determining
  which encoding characters are present